1. Field of the Invention
The present invention relates to D.C. excitation systems for A.C. multipolar dynamoelectric machines and is particularly concerned with brushless excitation systems arranged to produce variable pole numbers in the rotor windings of such machines.
2. Description of the Related Art
In many cases it is possible to vary the speed of a motor or the frequency of an alternator by varying the pole number. However, known arrangements for generating variable pole numbers require relatively complicated switching arrangements and/or require large numbers of slip ring connections.